1. Field of Invention
This invention relates to the recovery of light in projection systems that might otherwise be wasted.
2. Description of the Related Art
Projection displays work by projecting light onto, e.g. a screen. The light is arranged in patterns of colors or brightness and darkness, or both. The patterns are viewed by a viewer who assimilates them by associating the patterns with images with which the viewer may already be familiar, such as characters or faces. The patterns may be formed in various ways. One way is by modulating a beam of light with a stream of information.
Polarized light may be modulated by filtering it with polarized filters. An LCD imager may be used to perform the modulation, e.g. in LCD-type projection displays. LCD imagers may be, e.g. transmissive or reflective. The LCD imager may include pixels that may be modulated by altering their polarization to either match the polarization of the incident light or differ from it. The pixels will pass light, in general, if their polarization matches the polarization of the incident light.
If the polarization of the pixels is modulated with information, such as a pattern with which a viewer may be familiar, the information will be projected onto a screen as the presence or absence of light. When the pixels are modulated with information forming a pattern with which a viewer may be familiar, a viewer may recognize the pattern projected onto the screen.
Only half of the light from the source will be of the correct polarization to input to the LCD imager after the light is polarized. The other half will be of the incorrect polarization, and hence unusable directly. It would be desirable if light of the wrong polarization could be recovered and used.
Various schemes have been developed to convert the incorrectly polarized light to the correct polarization so that it may be used. The most common scheme uses an array of lenses and an array of polarization beam splitters (PBS). Another system involves the use of tapered light pipes and a PBS in which the polarization recovery is performed inside the light pipes. Such a system could be used with a 1:1 dual paraboloid coupling system.
In most of these systems, however, the output is collected at half the system etendue. The polarization recovery system then doubles the output etendue back to the system etendue. Since less light will be collected at half the system etendue, such a system is not as efficient as it could be. It would be desirable if the etendue could be maintained at its original size.
Colored light may also be used to illuminate imagers in projection displays. A color image may be produced by, e.g. spatially modulating several, e.g. three, colored beams of light incident on an imager and recombining them in desired proportions to produce an image. A range of colors may be produced by recombining the modulated beams in various proportions. A typical set of colored beams may be red, green, and blue (R, G, B), although other sets, such as, e.g. yellow, magenta, and cyan, may also be used.
If three colors are used, the three colored beams may be produced by three separate chips, such as three light emitting diodes (LED), each of which produces light in one of the colors. Such three-chip systems, however, may be mechanically and optically complex, and expensive. Systems having a single light source are often preferable to three-chip systems. It would be desirable to use a single chip to produce the light.
Light from a single light source, such as a single LED chip producing, e.g. white light, may used by splitting the light into three constituent color beams. Each beam is then spatially modulated by an imager and the three modulated beams are recombined to produce the desired color image, in the manner of the three-chip system. The beam from a single chip system may be split into three colors by using filters such as colored pixels or a color wheel. Such systems, however, may lose two-thirds of the total light to color filtering. It would be desirable if the light of the wrong color could be recovered and used.
Systems have been designed to ameliorate such filtering losses. One such system, produced by Koninklijke Philips Electronics N.V. (Philips), uses rotating prisms to scroll the beams across an imager. Red, green, and blue beams are produced by color separation filters. These beams form color bands on the imager and, with proper driver electronics, the appropriate pixels may be modulated according to the color bands' positions. The eye perceives a continuous color image if the colors are scrolled quickly enough.
A drawback of this system is that the etendue of the imager may be reduced by a factor of 3 to 5, depending on the tolerance of the system. This results in a reduction in light collection efficiency. A larger, more expensive chip may thus be required to compensate for the losses. It would be desirable if the etendue could be maintained, so that a larger chip was not necessary.
Texas Instruments, Inc. (TI) has a scrolling system that utilizes a spiral color wheel. The wheel scrolls color beams across an imager as in the Philips system. Some of the light reflected by the spiral color wheel is recaptured by reflecting it off the input surface of the light pipe. The light pipe has an input surface with an aperture to receive light focused onto the input surface by a reflector. The remainder of the input surface reflects the recaptured light back to the color wheel.
Since the aperture in the input surface is smaller than the available input surface, however, the brightness of the system is reduced, which increases the etendue of the output beam. In addition, some of the reflected light from the color wheel is lost through the input aperture. It would be desirable if the input surface were not limited in size to an input aperture.